Skewed Grab or Scale in Edit Mode
GG can actually extend edges beyond existing bounds. 

Either: drag slightly inwards to establish the edges you're sliding along, and then hit C or hold Alt while dragging outwards along the (now displayed) extended edges,

Or Go into the 'Adjust Last Operation' panel, and uncheck 'Clamp'

It's always worth checking operator options displayed at the bottom of the 3D View when you're getting to know new tools.. I'm still picking stuff up from there, even after using Blender for quite a long time.